Dynamics in host-guest complexes of molecular tweezers and clips

The dynamics in the host-guest complexes of the molecular tweezers 1 a,b and clips 2 a,b with 1,2,4,5-tetracyanobenzene (TCNB, 3) and tropylium tetrafluoroborate (4) as guest molecules were analyzed by temperature-dependent 1H NMR spectroscopy. The TCNB complexes of tweezers 1 a,b were found to be particularly stable (dissociation barrier: DeltaG(++)=16.8 and 15.7 kcal mol(-1), respectively), more stable than the TCNB complexes of clips 2 a,b and the tropylium complex of tweezer 1 b (dissociation barrier: DeltaG(++)=12.4, 11.2, and 12.3 kcal mol(-1), respectively). A detailed analysis of the kinetic and thermodynamic data (especially the negative entropies of activation found for complex dissociation) suggests that in the transition state of dissociation the guest molecule is still clipped between the aromatic tips of the host molecule. The 1H NMR analysis of the TCNB complexes 3@1 b and 3@2 a at low temperatures (T<-80 degrees C) showed that 3 undergoes fast rotation inside the cavity of tweezer 1 b or clip 2 a (rotational barrier: DeltaG( not equal)=11.7 and 8.3 kcal mol(-1), respectively). This rotation of a guest molecule inside the host cavity can be considered to be the dynamic equilibration of noncovalent conformers. In the case of clip complex 3@2 a the association and rotational barriers are smaller by DeltaDeltaG(++)=3-4 kcal mol(-1) than those in tweezer complexes 3@1 a,b. This can be explained by the more open topology of the trimethylene-bridged clips compared to the tetramethylene-bridged tweezers. Finally, the bromo substituents in the newly prepared clip 2 b have a substantial effect on the kinetics and thermodynamics of complex formation. Clip 2 b forms weaker complexes with (TCNB, 3) and tetracyanoquinodimethane (TCNQ, 12) and a more stable complex with 2,4,7-trinitrofluoren-9-ylidene (TNF, 13) than the parent clip 2 a. These results can be explained by a less negative electrostatic potential surface (EPS) inside the cavity and a larger van der Waals contact surface of 2 b compared to 2 a. In the case of the highly electron-deficient guest molecules TCNB and TCNQ the attractive electrostatic interaction is predominant and hence responsible for the thermodynamic complex stability, whereas in the case of TNF with its extended pi system, dispersion forces are more important for host-guest binding.     

The miscibility of poly(D,L-lactide-co-glycolide) with amphiphilic molecules and the interaction of their mixtures with DNA at air/water interface

The miscibility of poly(D,L-lactide-co-glycolide) (PLG) with three amphiphilic molecules and the interaction of the PLG/surfactant mixtures with DNA at air/water interface are investigated by pi-A isotherms, Brewster angle microscopy (BAM) and atomic force microscopy (AFM) techniques. The pi-A isotherms of the PLG mixtures with cationic C(12)AzoC(6)PyBr, and C(12)AzoC(6)N(CH(3))(3)Br, are quite different from the pi-A isotherm of pure PLG on water subphase. In contrast to the case, the pi-A isotherm of PLG mixed with nonionic C(12)AzoC(6)OPy is almost identical to the pure PLG except some increasing of molecular area. Similar phenomena are observed on DNA subphase. The in situ BAM and ex situ AFM observations demonstrate that the dispersion of PLG at air/water interface becomes good when it mixes with the two cationic surfactants, whereas quite poor due to the phase separation when it mixes with the nonionic amphiphilic molecule. Based on these results we conclude that the cationic surfactants can affect the conformation change of PLG at air/water interface and figure a well miscibility with polymer whereas the nonionic amphiphilic molecule presents poor miscibility. In addition, the even mixing of the PLG and the cationic surfactants is favorable for the adsorption to DNA more effectively.     

Characterization of Langmuir-monolayers of molecular clips by means of Brewster-angle-microscopy

Molecular clips are able to selectively bind electron deficient aromatic and aliphatic substrates, and these processes are usually investigated in dilute solutions of organic solvents. Caused by discrepancies between polar and hydrophobic groups, molecular clips are surface-active compounds and, in analogy to surfactants, they can form monomolecular films at the water surface. In this publication, we systematically investigated the self-association process and the phase-behaviour of three different molecular clips with the polar head groups -OCH₂COOH (a), -OCH₂COOEt (b), and -OCONHPh (c) by means of surface-pressure-area-isotherms and Brewster-angle-microscopy (BAM). We observed marked differences for all investigated surface-active compounds. The molecular surface areas of the three clips, determined from pressure-area-isotherms, could be traced back to the molecular diameters of the amphiphilic compounds. In several experiments we investigated the influence of diverse film compression and expansion steps. Hysteresis effects could be explained by different film morphologies. In a series of experiments we could show that the aromatic guest molecule 1,2,4,5-tetracyanobenzene (TCNB), which strongly binds to molecular clips, did not influence the phase diagrams and film structures.     

Synthesis and supramolecular properties of molecular clips with anthracene sidewalls

Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.     

Phase behavior and morphology of equimolar mixed cationic-anionic surfactant monolayers at the air/water interface: isotherm and Brewster angle microscopy analysis

The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic-anionic surfactants at the air/water interface were investigated by measurements of surface pressure-area per alkyl chain (pi-A) and surface potential-area per alkyl chain (DeltaV-A) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C(12) to C(16) were used to form mixed surfactant monolayers on the water subphase at 21 degrees C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the DeltaV-A isotherms were generally more sensitive than the pi-A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic-anionic surfactants were closely related to the alkyl chain lengths of the components.     

Orientational changes in dipalmitoyl phosphatidyl glycerol Langmuir monolayers

Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure-area (pi-A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 A as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.     

Interactions of L-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.     

Molecular organization of a water-insoluble iridium(III) complex in mixed monolayers

In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.     

Monolayer behavior of a pyridyl head-group-containing amphiphile and its miscibility with poly(D,L-lactide-co-glycolide) on different pH subphase

In this paper, we investigated the Langmuir film and Langmuir-Blodgett (LB) monolayer film of a nonionic amphiphilic molecule, 4-(6-p-pyridyloxyl)hexyloxyl-4'-dodecyloxylazobenzene (C(12)AzoC(6)Py) and its mixture with poly(D,L-lactide-co-glycolide) (PLG) at different subphase pH values (2.0, 2.6, 3.3, 4.4, and 6.5, respectively) by surface pressure-area (pi-A) isotherms, in situ interface Brewster angle microscopy (BAM), and ex situ atomic force microscopy (AFM). For pure C(12)AzoC(6)Py, its pi-A isotherms display a plateau when the subphase pH value is lower than 3.0. The pressure of the plateau increases with the decrease of pH until 2.0. Over the plateau, the pi-A isotherms become almost identical to the one under neutral conditions. The appearance of such a plateau can be explained as the coexistence of protonation and unprotonation of pyridyl head groups of the employed amphiphile. In contrast to the homogeneous surface morphology of pure C(12)AzoC(6)Py near the plateau by BAM observation, the surface in the case of its mixing with PLG exhibits a dendritic crystalline state under low surface pressure at subphase pH lower than 3.0. The crystalline state becomes soft and gradually melts into homogeneous aggregates with surface pressure increasing to a higher value than that of the plateau. Meanwhile, the hydrolysis of PLG in the mixture system at the interface has been affirmed to be restrained to a very large extent. And the PLG was believed to be compelled to the up layer of the LB film due to the phase separation, which is examined by AFM. Based on the experimental results, the corresponding discussion was also performed.     

Phase behavior of 2,3-disubstituted methyl octadecanoate monolayers at the air-water interface

The phase behavior of 2,3-disubstituted methyl octadecanoate monolayers at the air-water interface is studied by film balance and a Brewster angle microscope (BAM). The comparison of the surface pressure-molecular area (pi-A) isotherms with the corresponding BAM images provides information on the phase behavior of the monolayers. Variations in the phase behavior of different 2,3-disubstituted methyl octadecanoate monolayers can be correlated with the size of the headgroups, the interactions between the polar molecular moieties and the subphase, and the intermolecular interactions. The enlarging of the headgroups makes forming a condensed monolayer difficult for the molecules, even after introduction of substituents giving rise to the formation of hydrogen bonds between the molecules, which may balance the steric repulsion and stabilize the monolayers. Model calculations of the two-dimensional lattice structure of the 2,3-disubstituted methyl octadecanoates on basis of the pg and p1 space group are performed and correspond well with the experimental results.     

Langmuir monolayer properties of perfluorinated double long-chain salts with divalent counterions of separate electric charge at the air-water interface

The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,omega-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)2 : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (micro perpendicular)-area (A) isotherms] and morphological images of CnBP(FC14)2 monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)2 formed a stable monolayer on water at 298.2 K, where these pi-A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the pi-A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the pi-A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)2 molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)2 occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression-expansion cycle curve, and DeltaV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)2 and FC14 monolayers was also confirmed by FM and BAM images.     

Two-dimensional miscibility studies of alamethicin and selected film-forming molecules

Alamethicin (ALM), a 20-amino acid antibiotic peptide (peptaibol) from fungal sources, was mixed in Langmuir monolayers with six different surfactants: semifluorinated (F6H18, F10H19, F8H10OH, F6H10SH) and hydrogenated (C18SH and DODAC), aimed at finding appropriate molecules for ALM incorporation for nanodevice construction. Alamethicin-containing mixed monolayers were investigated by means of surface manometry (pi-A isotherms) and Brewster angle microscopy (BAM). Our results show that only semifluorinated alkanes can serve as an appropriate material since they form miscible and homogeneous monolayers with ALM within the whole concentration range. All the remaining surfactants, possessing polar groups, were found to demix with ALM. This effect was explained as being due to the existence of strong polar interactions between vertically oriented surfactant molecules, which tend to separate from horizontally oriented alpha-helices of the peptide. On the contrary, semifluorinated alkanes, lacking any polar group in their structure and bearing a large dipole moment, interact with ALM, also possessing a huge cumulative dipole moment. These dipole-dipole interactions between ALM and SFAs are more attractive than those between SFA molecules in their pure monolayers, causing the large ALM molecule, situated parallel to the interface, to be surrounded by SFA molecules in perpendicular orientation, leading to the formation of a highly organized binary mixed monolayer. BAM images of the ALM monolayer indicate that this peptide collapses with the nucleation and growth mechanism, like the majority of surfactants, which contradicts the model of ALM collapse by desorption, previously published in the literature.     

Nucleation and growth in the collapsed langmuir monolayers from semifluorinated alkanes

The 3D phase formation was monitored in relaxation experiments of the collapsed Langmuir monolayers of selected partially fluorinated tetracosanes, that is, F6H18, F8H16, and F10H14. To carry out these experiments, the classical method of surface manometry, such as pi-A isotherms registration and the molecular area-time dependencies, under quasi-static monitoring conditions were applied. The evolution of 3D structures at the water/air interface was observed with Brewster angle microscopy (BAM). The obtained data were interpreted according to the nucleation-growth-collision theory model. It occurred that, even though the investigated chemicals are not classical surfactants and do not possess any polar headgroup, their evolution from a 2D monolayer to 3D structures can be successfully modeled with the above-mentioned theory. The influence of the subphase temperature on the nucleation process is also discussed.     

Monolayer characteristics of mixed octadecylamine and stearic acid at the air/water interface

Mixed monolayers of stearic acid (SA) and octadecylamine (ODA) at the air/water interface were investigated in this article. The miscibility of the two compounds was evaluated by the measurement of surface pressure-area per molecule (pi-A) isothems and the direct observation of Brewster angle microscopy (BAM) on the water surface. The two compounds were spread individually on the subphase (method 1) or premixed first in the spreading solvent and then cospread (method 2). The effect of spreading method on the miscibility of the two compounds was also studied. The results show that the mixed monolayers prepared by method 1 cannot get a well-mixed state. The isotherms of mixed monolayers preserve both characteristics of SA and ODA and exhibit two collapse points. The calculated excess surface area is very small. Besides, distinguished domains corresponding to those of pure SA and ODA can be inspected from the BAM images. Such results indicate that SA and ODA cannot get a well-mixed phase via 2-dimensional mixing. On the contrary, in the mixed monolayer prepared by cospreading, the two compounds exhibit high miscibility. In the pi-A isotherms, the individual characteristics of SA and ODA disappear. The calculated excess area exhibits a highly positive deviation which indicates the existence of special interaction between the two compounds. The low compressibility of isotherm implies the highly rigid characteristic of the mixed monolayer. which was also sustained by the striplike collapse morphology observed from the BAM. The rigid characteristic of SA/ODA mixed monolayer was attributed to the formation of "catanionic surfactant" by electrostatic adsorption of headgroups of SA and ODA or to the formation of salt by acid-base reaction.     

Molecular organization in two-dimensional films of liquid I. Langmuir films of binary mixtures of liquid crystals with a crystalline mixtures terminal cyano group

The Langmuir films of two liquid crystal materials, 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT), and of their mixtures have been studied by recording surface pressure-area isotherms and Brewster angle microscopy (BAM) images. The pure liquid crystals revealed very different characters of the surface pressure-area isotherms indicating different organization of the molecules and different molecular interactions in the monolayer at the water-air interface. The surface pressure-area isotherms of Langmuir films formed from 8CB/5CT mixtures give evidence for phase separation of the components over the whole range of molar fractions. Similar conclusions have been drawn on the basis of BAM image analysis.     

Molecular organization in two-dimensional films of liquid I. Langmuir films of binary mixtures of liquid crystals with a crystalline mixtures terminal cyano group

The Langmuir films of two liquid crystal materials, 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT), and of their mixtures have been studied by recording surface pressure-area isotherms and Brewster angle microscopy (BAM) images. The pure liquid crystals revealed very different characters of the surface pressure-area isotherms indicating different organization of the molecules and different molecular interactions in the monolayer at the water-air interface. The surface pressure-area isotherms of Langmuir films formed from 8CB/5CT mixtures give evidence for phase separation of the components over the whole range of molar fractions. Similar conclusions have been drawn on the basis of BAM image analysis.     

Influence of temperature and alkyl chain length on phase behavior in Langmuir monolayers of some oxyethylenated nonionic surfactants

We study the surface phase behavior in Langmuir monolayers of a series of nonionic surfactants of the general formula CnE1 with n=14, 16, and 18 by film balance and Brewster angle microscopy (BAM) over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressure-area (pi-A) isotherms indicates a first-order phase transition in the coexisting state between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the air-water interface. The formation of bright two-dimensional (2D) LC domains in a dark background visualized by BAM further confirms this observation. In addition to the cusp point at the onset of the LE-LC coexistence state, another cusp point followed by a small plateau is observed for the C14E1 and C18E1 monolayers, indicating a second phase transition between two condensed phases of different compressibility and tilt orientation of the molecules. This unusual two-step phase transition is explained by the Ostwald step rule. The C16E1 and C18E1 monolayers show a kink in their respective isotherms, after which the surface pressure increases steeply with only a little decrease in the molecular area, suggesting that the molecules undergo a transition from a tilted to an almost vertical orientation with respect to the water surface. The thermodynamic parameters for the condensation of the molecules in the LE-LC coexistence state were calculated by employing the 2D Clapeyron equation. The temperature coefficient of the critical surface pressure dpi(c)/dT values shows a decreasing trend from C14E1 to C18E1, suggesting that the condensation process becomes less and less prone to thermal perturbation as the chain length increases. For all the amphiphiles, the DeltaH values are found to be negative, suggesting an exothermic nature of condensation. The negative DeltaS values obtained from the relation DeltaH/T probably come from the restriction on the rotational and translational motion of the molecules constrained in a confined area in the LE-LC transition region.     

Surface characterization of poly(4-vinylpyridine) quaternized with tetradecyl bromide: effect of the degree of quaternization

The surface behavior of poly(4-vinylpyridine) quaternized with tetradecyl bromide (P4VPC(14)) as function of the quaternization degree was studied. The percentage of vinylpyridine moieties quaternized was found to be 35 to 75%. Surface pressure-area isotherms (pi-A) at the air-water interface were determined. The polymer monolayers show particular shapes at different quaternization degrees. In order to get information about the hydrophobicity degree of the polymeric systems, the surface energy (SE), and their dispersion and polar contributions, gamma(D) and gamma(P) respectively, measurements of the contact angle (CA) with water, bromobenzene, and cis-decalin were performed. The results obtained are dependent on the quaternization percentage of the functionalized polymers. At high quaternization degree, hysteresis in the pi-A diagrams was observed. The change in the P4VPC(14) molecular organization in the monolayer was also investigated during the compression and expansion processes. During these processes the monolayer was monitored by Brewster angle microscopy (BAM). For several cycles, all compression curves and all expansion curves showed a common intersection point. We have tried to describe the P4VPC(14) molecular organization at the air-water interface by molecular dynamic simulation (MDS). The results are also discussed in terms of the effect of the counterions and water on the stability of the system over the more packed region.     

The effect of temperature on food emulsifiers at fluid-fluid interfaces

Heat-induced interfacial aggregation of a whey protein isolate (WPI) with a high content of beta-lactoglobulin (>92%), previously adsorbed at the oil-water interface, was studied by means of interfacial dynamic characteristics performed in an automatic drop tensiometer. Protein concentration in aqueous bulk phase ranging between 1x10(-1) and 1x10(-5) % wt/wt was studied as a variable. The experiments were carried out at temperatures ranging from 20-80 degrees C with different thermal regimes. During the heating period, competition exists between the effect of temperature on the film fluidity and the increase in mechanical properties associated with the interfacial gelation process. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) previously adsorbed at the oil-water interface, was studied by interfacial dynamic characteristics (interfacial tension and surface dilational properties). The temperature, ranging between 40 and 2 degrees C, and the lipid concentration in aqueous oil phase, ranging between 1x10(-2) and 1x10(-4) % wt/wt, were studied as variables. Significant changes in interfacial dynamic characteristics associated with interfacial lipid crystallisation were observed as a function of lipid concentration in the bulk phase. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) at the air-water interface, was studied by pi-A isotherms performed in a Langmuir trough coupled with Brewster angle microscopy (BAM). A condensation in monoglyceride monolayers towards lower molecular area was observed as the temperature decreased. This effect was attributed to lipid crystallisation at lower temperatures. BAM images corroborated the effect of temperature on the monolayer structure, as a function of the monoglyceride type.     

Effect of hydrocarbon chain and pH on structural and topographical characteristics of phospholipid monolayers

Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.